Remove dead code

Summary:
Setting the layout height to be the height of one of the layout items is
both pointless, it will be calculated from all child items
implicitHeight anyway, and wrong as the heading/textField could be
bigger.

CCBUG: 365295

Test Plan: Opened it, looked the same

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2241